Butter an ovenproof pan large enough to take the fillets without overlapping. Cover the base of the pan with the shallots, place the fillets of brill on top, and season. Add the red wine and fish stock, cover with buttered paper or foil and bring to just below boiling point. Transfer to the warm oven (the oven should not be so hot that it makes the liquid boil) and
Put the stock in a saucepan and reduce by about half, then gradually add the cubes of butter, beating with a small whisk until all the butter has melted. Do not allow the sauce to boil again once the butter has been added.